1. Field of the Invention
The present invention relates to a guidance apparatus for operating a sodium-sulphur battery. In particular, the present invention relates to a guidance apparatus used to efficiently operate a sodium-sulphur battery that structures a hybrid system for supplying power with combination with a generator whose output changes, such as a wind turbine.
2. Description of the Related Art
As a main constituting unit of a battery energy storage system for leveling a power demand (load) or a backup power apparatus for natural disasters, a sodium-sulphur battery is put into practical use. The sodium-sulphur battery is a secondary battery in which molten metal sodium as a cathode active material and melt sulphur as an anode active material are arranged separately from each other using a β-alumina solid electrolyte having selective permeability toward sodium ions. In the sodium-sulphur battery, the molten metal sodium discharges electrons to be sodium ions, the sodium ions are permeated through a solid electrolyte and are moved to the anode side, sodium polysulfide is generated in reaction to electrons supplied from sulphur and an external circuit, and discharge operation is performed. On the contrary to the discharge operation, reaction to generate sodium and sulphur from sodium polysulfide enables charge operation (with respect to the sodium-sulphur battery, refer to, e.g., JP-A-2003-288950 and JP-A 2000-182662).
The following two points may be taken up as a major factor to be taken into consideration for operating the sodium-sulphur battery.
Firstly, the remaining energy (or discharge capacity) should be properly managed. In the sodium-sulphur battery, if sodium polysulfide is formed at the anode side and the concentration of sodium ion at the cathode side becomes short due to overdischarge, the charge/discharge operation after that becomes impossible. Further, if the solid electrolyte is damaged and the active substance leaks due to overdischarge, the charge/discharge operation thereafter becomes impossible. Thus, the charge can not be continued if the remaining energy exceeds the end of the charge which is detectable with a voltage, and, similarly, the discharge can not be continued if the capacity exceeds the end of the discharge. Thus, if the capacity suddenly reaches the end of charge or discharge, the above-mentioned apparatus can not function as a battery energy storage system (battery). Accordingly, it is recognized that the management of the remaining energy is quite important.
Secondly, the module temperature should be controlled within a predetermined range (about, 280 to 360° C.). Preferably, the sodium-sulphur battery is operated at a high temperature not less than 280° C. in order to efficiently perform the charge/discharge operation because of temperature characteristics of a sodium ion conductivity of the β-alumina solid electrolyte. On the other hand, various members constituting the battery have a limit in the thermal endurance, and the module temperature of the sodium-sulphur battery is thus limited. Therefore, it is important to operate the sodium-sulphur battery within the above-mentioned predetermined temperature range.